Soil microbial communities play a critical role in the decomposition of organic matter and cycling of carbon (C) and nitrogen (N). In a context of global change and decline in biodiversity, understanding how microbial communities interact with tree diversity under different conditions of water availability is an essential issue to maintain soil functions and adequately manage forests. Our objective was to characterize how tree species richness and manipulated water availability affect the functional diversity of microbial communities and soil C and N pools. Two experiments within the TreeDivNet platform allowed us to study the interaction between tree species richness and water deficit, namely the ORPHEE experiment in Bordeaux (France), and the IDENT experiment in Sault-Sainte-Marie (Ontario). Plantations of single species monocultures and species mixtures were established in 2008 and 2013, respectively, and were subject to treatments of either irrigation or rainfall exclusion. At Bordeaux, we investigated a gradient of 1 to 3 species (Betula pendula, Pinus pinaster, and Quercus robur) and in Sault-Sainte-Marie, a gradient of 1 to 4 species (Betula papyrifera, Pinus strobus, Picea glauca, Quercus rubra). Mineral soil (0-10 cm) was sampled and analyzed to assess C and N concentrations, and to assess functional diversity of the microbial community by the Microresp method.
Results/Conclusions
Contrary to expectations, soil C and N concentrations did not increase with water irrigation, nor with species richness, although these concentrations were influenced by species identity. Decrease in soil microbial activity with drought was significant in Sault-Sainte-Marie, but in Bordeaux the effect of drought depended on species richness. The interactions species richness x water deficit were consistently significant for most variables (including soil N), indicating that microbial communities were particularly sensitive to tree diversity and water deficit, even if these factors did not influence the functional diversity of microorganisms. The regions of Aquitaine and Ontario in particular experience changes in seasonal precipitation and occurrence of drought, and tree diversity in plantations may have important impacts on soil microbial communities and hence, impacts on long-term soil C and N sequestration.